1. Field of the Invention
The present invention relates to a structure of a liquid crystal display device (LCD) and a manufacturing method thereof. More specifically, the present invention relates to a structure of an LCD in which a side edge of a common electrode disposed on the entire surface of an upper substrate and a pad disposed on the lower substrate facing the upper substrate are not overlapped, and the present invention also relates to a method of manufacturing such an LCD.
2. Description of the Background Art
The CRT (Cathode Ray Tube), a widely used display device, is being replaced by the thin flat panel display device because the thin flat panel display device is thinner and lighter than the CRT so it can be located any place. Research has been focused on the development of liquid crystal display devices because of their high resolution and fast response time which is suitable for displaying motion picture images. Furthermore, the active panel including an active switching element such as a thin film transistor (TFT) is being applied to the LCD much more frequently and easily.
A liquid crystal display device works by using polarization and optical anisotrophy of a liquid crystal material. By controlling the orientation of rod-shaped liquid crystal molecules through a polarization technique, transmission and interception of a light through the liquid crystal are achieved because of the anisotrophy of the liquid crystal material. This principle is used in the liquid crystal display device. Active matrix liquid crystal displays (AMLCDs) having TFTs arranged in a matrix pattern and pixel electrodes connected to the TFTs provide high quality images are widely used.
The structure of a conventional AMLCD will now be described referring to FIG. 1 which shows the perspective view of the conventional AMLCD and FIG. 2 which shows a cross-sectional view of the conventional AMLCD cut along line II—II in FIG. 1.
Generally, a liquid crystal display device includes two panels 3 and 5 each having several elements and arranged to face each other with a liquid crystal material 10 located between the two panels 3, 5. The first panel, a color filter panel 3, includes a sequential arrangement of red, blue and green color filters 7 on a first transparent substrate 1a at pixel positions arranged in a matrix pattern. Among these color filters 7, black matrixes 9 are arranged in a lattice pattern. The black matrixes 9 prevent mixing of adjacent colors. A common electrode 8 is located on the color filters 7. The common electrode 8 is one electrode forming the electric field for driving the liquid crystal material 10.
The second panel, an active panel 5, includes pixel electrodes 47 arranged in a matrix pattern and formed on a second transparent substrate 1b. The pixel electrode 47 is the other electrode forming the electric field for driving the liquid crystal material 10. Scan bus lines 13 are arranged along the column direction of the pixel electrodes 47 and data bus lines 23 are arranged along the row direction of the pixel electrodes 47. At a corner of a pixel electrode 47, a TFT for driving the pixel electrode 47 is formed. A gate electrode 11 of the TFT is connected to a scan bus line 13 which is also referred to as a gate line. A source electrode 21 of the TFT is connected with a data bus line 23 which is also referred to as a source line. A drain electrode 31 of the TFT is connected to the pixel electrode 47. A semiconductor layer 33 is disposed between the source electrode 21 and the drain electrode 31. The source electrode 21 and the semiconductor layer 33 and the drain electrode 31 and the semiconductor layer 33 are ohmic contacted, respectively. Additionally, a gate pad 15 is formed at the end portion of each gate line 13 and a source pad 25 is formed at the end portion of each source line 23. A gate pad terminal 57 and a data pad terminal 67 are formed on the gate pad 15 and the source pad 25, respectively. Each of the pad terminals 57 and 67 contact the voltage supplying layer (not shown) and is supplied with the signal and data voltage respectively, from the driving IC.
As a signal voltage applied to the gate pad 15 via the gate pad terminal 57 is applied to the gate electrode 11 through the gate line 13, the TFT of the corresponding gate electrode 11 is in an ON state. Then the source electrode 21 and the drain electrode 31 of the TFT 19 are electrically connected so that the electrical picture data applied to the source pad 25 is sent to the drain electrodes 27 through the source line 23 and the source electrode 21. Otherwise, no signal voltage is applied to the gate pad 15, then the source electrode 21 and the drain electrode 31 of the TFT are electrically isolated. Therefore, controlling the signal voltage of the gate electrode 11 determines whether the picture data is applied to the drain electrode or not. That is, the TFT 19 acts as a switching element. A gate insulating layer 17 is inserted between the layer including the gate electrode 13 and the layer including the source electrode 23 to isolate them electrically. A passivation layer 37 is formed on the layer including the source line 23 to protect all of the elements of the LCD.
The color filter panel 3 and the active panel 5 are bonded together so as to face each other and so as to be spaced apart by a certain distance which is referred to as a cell gap. Liquid crystal material 10 fills the cell gap and the edges of the bonded panels are sealed with a sealant 81 such as an epoxy to prevent the liquid crystal material 10 from leaking. After the liquid crystal material 10 is injected between the color filter panel 3 and the active panel 5 which is joined together by the sealant 81, portions of the color filter panel 3 covering the gate pad 15 and the source pad 25 are removed in order to expose the pads 15 and 25 for connection to outputs of the driver IC so that a liquid crystal panel of an AMLCD is completed, as shown in FIGS. 1 and 2.
The disadvantage of the conventional LCD panel mentioned above will now be described referring to FIG. 3 which shows how a pad portion is shorted to the common electrode by a remaining conductive material. During the cutting process, a portion of the pad terminals 57 and 67 are essentially overlapped with the cutting edge of the color filter panel 3. By this mechanical cutting, therefore, conductive materials 77 such as Indium Tin Oxide (ITO) of the common electrode 8 which is formed on the entire surface of the upper panel 3 are left over or remain between the conductive common electrode 8 and the conductive pad terminal 57, as shown in FIG. 3. In this case, the gate pad terminal 57 of the lower panel 5 is electrically connected to the common electrode 8 of the upper panel 3 via the remaining conductive material 77. As a result, there are disadvantages that the conventional LCD panel is often shorted out because of the remaining conductive material 77 connected the gate pad terminal 57 and the common electrode 8. In this case, the LCD does not work properly.